The present invention relates to a variable gain amplifier. More particularly, it relates to a variable gain amplifier enhanced in input and output characteristics, for use in amplification of radio frequency signals in various wireless communication appliances.
In this kind of amplifier known hitherto, for example, at the input side of a semiconductor element for amplifying, a semiconductor element for signal attenuation capable of varying the conductive state by a control voltage from outside is provided in series between a radio frequency input line to the semiconductor element for amplifying and the ground, and the input signal to the semiconductor element for amplifying can be attenuated, so that the gain may be variable (for example, Japanese Unexamined Patent Publication No. 2001-237650, pages 3–4, FIG. 1).
The gain of the amplifier can be varied not only by installing a gain variable circuit as mentioned above, but also, for example, by cutting off the supply voltage of the amplifier, for a sufficient signal input, so as to obtain a minimum gain.
At the input side of the amplifier mentioned above, an input impedance matching circuit for matching the impedance between the circuit of the preceding stage and the amplifier is often disposed, and at the output side, further, an output impedance matching circuit for matching the impedance between the circuit of the later stage and the amplifier is disposed.
Such impedance matching circuit is optimized, usually assuming a case of input of a feeble signal, when the gain is not varied, in other words, when the gain of the amplifier is at maximum, and a favorable characteristic is obtained in the voltage standing wave ratio (VSWR) at the input end and output end of the amplifier. For example, FIG. 6(A) shows an example of measurement of VSWR at the input end when the gain is not varied and when the gain is varied in the amplifier of the configuration shown in the above publication, and FIG. 6(B) shows an example of measurement of VSWR at the output end when the gain is not varied and when the gain is varied in the amplifier of the configuration shown in the above publication, in which it is known that the measurement of VSWR is favorable when the gain is not varied in both cases. In FIG. 6(A), X01 shows a point where the measurement of VSWR at the input end when the gain is not varied is 1.78, and Y01 shows a point where the measurement of VSWR at the input end when the gain is varied is 9.89. In FIG. 6(B), X02 shows a point where the measurement of VSWR at the output end when the gain is not varied is 1.69, and Y02 shows a point where the measurement of VSWR at the output end when the gain is varied is 4.23.
On the other hand, when the gain is varied, that is, in the case the gain of the amplifier is minimum, in the amplifier of the configuration shown in the above publication, the semiconductor for signal attenuation is turned ON (in conducting state), and the input and output impedances of the amplifier are significantly difference from those when the gain is not varied, and therefore if the impedance matching circuit is provided each at the input and output end, since these impedance matching circuits are optimized when the gain of the amplifier is maximum, and therefore VSWR when the gain is varied is extremely degraded as shown in FIG. 6(A) and FIG. 6(B).
As a result, it leads to degrading of characteristic of the filter connected in the front stage or rear stage of the amplifier, thereby extremely degrading the reception performance of the wireless communication appliance.
In the case of the amplifier designed to vary the gain by presence or absence of power supply without installing a circuit for varying the gain, although it is an advantage that the power consumption of the wireless communication appliance can be saved by cutting off the power supply, even in this case, since the input and output impedances of the amplifier are significantly different between when the power is supplied and when the power supply is cut off, VSWR at the input and output ends of the amplifier when the power supply is cut off is worse as compared when the power is supplied, and this is the same problem as in the amplifier having the circuit for varying the gain.
Further, in the amplifier for varying the gain by presence or absence of supply of supply voltage, since the gain when varying the gain (when the power supply is not cut off is determined nearly by the isolation between input and output of the semiconductor element for signal amplification, it is impossible to set arbitrarily the gain attenuation amount, that is, the difference in gain when the gain is not varied (the power is supplied) and when the gain is varied (the power is not supplied), which is not convenient for use.